The Effect Of Vegetation Recovery Of Two Kinds Of Site Preparation In Northern Shaanxi Loess Area

The ecological environment of northern Shaanxi loess area is fragile with the serious soil erosion, and it is one of the key areas of vegetation recovery. With the implementation of ecological construction projects, remarkable improvments of vegetation recovery were achieved, but the research and summary of theory and the technical system are still not perfect. The influencing factors of ecological restoration were analyzed with the previous research results, and a hypothesis concerning the material and energy balance of ecological recovery was built. The vegetation restoration effects of artificial Pinus tabulaeformis forest with two kinds of soil preparation were analysised.The engineering technology effects of the fish-scale pit and level bench were comprehensively researched. The results showed that during July and August, the average soil water contents inside the fish-scale pits were below the levels of external slope, and the effect of each fish-scale pits was different, which was mainly in the runoff periods. As a result, the catchment effects of each fish-scale pits were influenced by layout mode, construction technology and other factors.80 specification was suitable for the steep and very-steep slopes of semi-sunny, however 80 or 60 specifications were suitable for all the gradual slopes.The average soil moisture of soil layer of 0-160 cm at the end of rainy season was 5.43%, while that was only 3.87%at the end of arid season. It was suggested that recharge capacity of soil moisture during rainy season is small. All the nutrient substances of alkali-hydrolyzable nitrogen, available phosphorus, available potassium, organic matter and total nitrogen in soil have the property of accumulating in surface soil, especially the nutrients in the soil layer of 0-10cm. However, neither total phosphorus nor total potassium in all soil layers has this property. The soil of every site is Partial alkaline with the PH value greater than eight. The average capacity of Total phosphorus, total potassium and available potassium in the soil layer of 0-100cm is medium, while that of organic matter, total nitrogen, alkali-hydrolyzable nitrogen and available phosphorus is low or even extremely low.Within the pine plantation, the optimal growth distance was 2.8 m, which of 0-2cm lower diameter size was 4.0m, but it did not change obviously with the diameter size increasing. As the spacing increased, the number of neighbor trees became more and the total intraspecies competition intensified, but the trend of intensified became weaker. Model of the total competition intensity and interference zone was:Cl=a+bL-1. The competition intensity decreased gradually with the DBH increasing, the model was Cl=aD-b. The competition between the pines was greatest among trees with diameters of up to 2 cm. For sample trees, the order of Nitrogen content is leaf, fruit, litter, humus, branch, bark, root, root head, stem;and that of Phosphorus content is humus, fruit, litter, branch, leaf, stem, root, bark, root head; and that of Potassium content is humus, leaves, roots, branches, litter, bark, fruit, root head, stem; and that of Carbon content is leaf, bark, branch, root head, fruit, root, stem, litter, humus. The plot with the highest Carbon reserve is at the gully slope slope, followed by the plot with the low-density by the level bench. The carbon stocks of plots with the fish-scale and without site preparation are less than the plot with the level bench at the same density, and that with high density is less than low density the density by the level bench too. The tree height, DBH and volume growth were lower than the average of the local pine forests. The growths of DBH and tree height entered the mature stage, while the volume growth is in the accelerated growth period.Therefore, it is proposed that the measure of level bench contributes to the formation of low-quality and benefit forest in the semi-arid loess region through weaken the self-thinning of forest plantation which due to homogenization of competitiveness of individual trees, when precipitation is insufficient and density of plantation is too high. Fish-scale pit is site preparation that human formation and expansion could form concave-shaped micro-topography, which could make precipitation resources to re-distribute, so fish-scale pit is recommended in the area.